A conventional servo controlling method for compensating a following delay of a servo system due to influence from friction when a feed axis of a machine tool reverses its moving/rotating direction will be described with reference to FIG. 7 that is a block diagram of the servo controlling device described in Japanese Patent Application No. H08-353627 (Japanese Unexamined Patent Publication No. H10-177407) applied by the present applicant.
In FIG. 7, pulse counter 5 counts output pulses 4 outputted from pulse generator 3, and calculates and outputs a position feedback 6 of servo motor 1. F/V converter 7 calculates and outputs a speed feedback 8 of the servo motor based on the output pulses 7 of the pulse generator 3.
A deviation between a position instruction 10 outputted from NC device 9 and the position feedback 6 is multiplied by a position loop gain Kp by multiplier 11, whereby a speed target value 12 is obtained. Integrator 14 integrates values that are obtained by multiplying a deviation between the speed target value 12 and speed feedback 8 by an integration gain 1/Ti or integration gain 1/Tih by multiplier 13 or multiplier 27. Furthermore, a deviation between a value resulting from integration by the integrator 14 and the speed feedback 8 is multiplied by a speed loop gain Kv by multiplier 15, whereby a torque instruction value 16 is obtained. Current amplifier 17 outputs a current 18 corresponding to the torque instruction 16 to the servo motor 1.
Compensator 26 monitors the position instruction 10 outputted from the NC device 9, estimates a quadrant change timing by taking the position instruction 10 and the delay of the feed axis of the machine tool into account, and outputs a changeover signal 33 for changing the integration gain to be multiplied by the speed target value 12 from the integration gain 1/Ti to the integration gain 1/Tih.
FIG. 8 shows an equivalent control block diagram of this compensator 26. The compensator 26 comprises multipliers 11, 15, and 27, integrator 28, and change control part 32.
The multiplier 11 multiplies the position instruction 10 by a position loop gain Kp. The multiplier 27 multiplies the output of the multiplier 11 by the integration gain 1/Tih. The integrator 28 integrates the output of the multiplier 27. The multiplier 15 multiplies the value integrated by the integrator 28 by a speed loop gain Kv to calculate an equivalent torque instruction 29. The changeover control part 32 detects a quadrant change based on the position instruction 20, and outputs a changeover signal 33 so that an integration gain by which the speed target value 12 is multiplied becomes an integration gain 1/Tih. When the equivalent torque instruction 29 reaches a torque at which predetermined compensation is terminated, a changeover signal 33 is outputted so that the integration gain by which the speed target value 12 is multiplied becomes an integration gain 1/Ti.
FIG. 9 shows torque instruction and speed feedback waveforms (a) when the conventional method is used when changing a quadrant, torque instruction and speed feedback waveforms (b) in a case where following delay compensation is not applied, and an ideal speed feedback waveform (c). By the abovementioned method, the integration gain 1/Tih of the multiplier 27 is made larger than the integration gain 1/Ti of the multiplier 13, a torque instruction is quickly generated by which the integration gain 1/Ti of the speed loop is changed to the integration gain 1/Tih only during a period (d) in which the shaft stops to cancel static friction, a period (e) in which the shaft stops in the case where automatic following delay compensation is not applied is shortened to the period (d), whereby the speed feedback waveform is made closer to the ideal waveform (c).
FIG. 7 shows an example of an I-P controlling method, however, there is also a case using a P-I controlling method.
In the conventional method which comprises an equivalent control block and uses compensation conditions obtained from the equivalent control block, the equivalent control block and a control system which drives the feed axis may be different in operation from each other. In such a case, for example, as shown in FIG. 10, when the position/speed feedback changes based on a judgement of the equivalent control block while the control system which drives the feed axis uses the integration gain 1/Tih of the integrator 27 during shaft stoppage, the control system becomes unstable, and the torque instruction vibrates or diverges.
Furthermore, the resolving power of the position/speed feedback is low, torque instruction changes due to changes in position/speed feedback become greater, so that a difference in operation between the equivalent control block and the control system which drives the feed axis becomes conspicuous.
In an arc instruction, the torque instruction 16 when the feed axis reverses differs depending on the cutting rate. FIG. 11 shows examples of torque instructions when cutting is carried out with the same arc radius at arc instruction feed speeds of F1000 mm/min and F10000 mm/min without following delay compensation. (a) and (b) in FIG. 11 show torque instruction values when the shaft starts moving in a reverse direction at F1000 mm/min and F10000 mm/min, respectively, wherein the torque instruction value differs depending on the speed. In the conventional method, the torque instruction value is fixed regardless of the speed, and as shown in FIG. 12, the value of the equivalent torque instruction 29 for terminating compensation is also fixed. Therefore, only a single, narrow-range speed can be accommodated, so that compensation effects at an out-of-range speed become insufficient or excessive.
The abovementioned conventional controlling method has the following problems.
(1) When the position/speed feedback changes while the control system for driving the feed axis uses the integration gain 1/Tih during shaft stoppage, the control system becomes unstable and the torque instruction vibrates or diverges.
(2) Only a single, narrow-range speed can be accommodated, and at an out-of-range speed, compensation effects become insufficient or excessive, and the accuracy of loci following control lowers.